The present invention relates to a data recording cartridge such as a magnetic disk cartridge and the like inclusive of a flexible magnetic recording medium, and more specifically, to a data recording cartridge and flexible magnetic recording medium having high reliability even at high temperatures. The data recording cartridge referred to in the present invention is a cartridge having a tape-shaped, disk-shaped or card-shaped magnetic recording medium, optical recording medium or semiconductor recording medium accommodated in a cartridge case to which and from which data from a computer or the like is capable of be magnetically, optically or electrically recorded and reproduced. Further, the flexible magnetic recording medium is a tape-shaped, disk-shaped or card-shaped recording medium composed of a flexible substrate such as a plastic film having a magnetic thin film formed thereon which is capable of magnetically recording data.